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Unix-like operating system

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Diagram of the relationships between the major Unix-like systems
Diagram of the relationships between
the major Unix-like systems
A Unix-like (sometimes referred to as UN*X or *nix) operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification.

There is no standard for defining the term, and some difference of opinion is possible as to the degree to which a given OS is "Unix-like".

The term can include free and open source operating systems inspired by Bell Labs’ Unix or designed to emulate its features, commercial and proprietary work-alikes, and even versions based on the licensed UNIX source code (which may be sufficiently "Unix-like" to pass certification and bear the "UNIX" trademark).


The Open Group owns the UNIX trademark and administers the Single UNIX Specification, with the "UNIX" name being used as a certification mark. They do not approve of the construction "Unix-like", and consider it a misuse of their trademark. Their guidelines require "UNIX" to be presented in uppercase or otherwise distinguished from the surrounding text, strongly encourage using it as a branding adjective for a generic word such as "system", and discourage its use in hyphenated phrases.

Other parties frequently treat "Unix" as a genericized trademark. Some add a wildcard character to the name to make a euphemistic abbreviation like "Un*x" or "*nix", since Unix-like systems often have Unix-like names such as AIX, HP-UX, IRIX, Linux, Minix, Ultrix, and Xenix. These patterns do not literally match many system names, but are still generally recognized to refer to any UNIX descendant or work-alike system, even those with completely dissimilar names such as Solaris or FreeBSD.

In 2007, Wayne R. Gray sued to dispute the status of UNIX as a trademark, but lost his case, and lost again on appeal.

Also in 2007, the Open Group reached a binding legal agreement to prevent the German University of Kassel from using "UNIK" as its short form name.


"Unix-like" systems started to appear in the late 1970s and early 1980s. Many proprietary versions, such as Idris (1978), Coherent (1983), UNOS (1983), and UniFlex (1985), aimed to provide businesses with the functionality available to academic users of UNIX.

When AT&T later allowed commercial licensing of UNIX in the 1980s, a variety of proprietary systems were developed based on it, including AIX, HP-UX, IRIX, SunOS, Tru64, Ultrix, and Xenix. These largely displaced the proprietary clones. Growing incompatibility between these systems led to the creation of interoperability standards, including POSIX and the Single UNIX Specification.

Meanwhile, the GNU Project was launched in 1983 with the goal of making GNU, an operating system which all computer users could freely use, study, modify, and redistribute. Various "Unix-like" operating systems developed alongside GNU, frequently sharing substantial components with it (leading to some disagreement about whether they should be called "GNU" or not). These primarily served as low-cost and unrestricted substitutes for UNIX, and include 4.4BSD, Linux, and Minix. Some of these have in turn been the basis for commercial "Unix-like" systems, such as BSD/OS and Mac OS X. Notably, Mac OS X 10.5 and Mac OS X 10.6 running on Intel Macs are certified under the Single UNIX Specification.

The various BSD variants are notable in that they are in fact descendants of UNIX, developed by the University of California at Berkeley with UNIX source code from Bell Labs. However, the BSD code base has evolved since then, replacing all of the AT&T code. Since the BSD variants are not certified as compliant with the Single UNIX Specification (except for Mac OS X 10.5 Leopard and Mac OS X 10.6 Snow Leopard), they are referred to as "UNIX-like".


Dennis Ritchie, one of the original creators of Unix, has expressed his opinion that Unix-like systems such as Linux are de facto Unix systems. Eric S. Raymond and Rob Langley have suggested that there are three kinds of Unix-like systems:

Genetic UNIX

Those systems with a historical connection to the AT&T codebase. Most (but not all) commercial UNIX systems fall into this category. So do the BSD systems, which are descendants of work done at the University of California, Berkeley in the late 1970s and early 1980s. Some of these systems have no original AT&T code but can still trace their ancestry to AT&T designs.

Trademark or Branded UNIX

These systems—largely commercial in nature—have been determined by the Open Group to meet the Single UNIX Specification and are allowed to carry the UNIX name. Most such systems are commercial derivatives of the System V code base in one form or another, although Apple Mac OS X 10.5 and later is a BSD variant, and has been certified, and a few certified systems (such as IBM z/OS) earned the trademark through a POSIX compatibility layer and are not otherwise inherently Unix systems. Many ancient UNIX systems no longer meet this definition.

Functional UNIX

Broadly, any Unix-like system that behaves in a manner roughly consistent with the UNIX specification; more specifically, this can refer to systems such as Linux or Minix that behave similarly to a UNIX system but have no genetic or trademark connection to the AT&T code base. Most free/open-source implementations of the UNIX design, whether genetic UNIX or not, fall into the restricted definition of this third category due to the expense of obtaining Open Group certification, which costs thousands of dollars, not being commercially necessary.

Compatibility layers

Some non-Unix-like operating systems provide a Unix-like compatibility layer, with variable degrees of Unix-like functionality.

  • IBM z/OS's POSIX compatibility layer is sufficiently complete to be certified as trademark UNIX.
  • Cygwin and MSYS both provide a reasonably complete GNU environment, sufficient for most common open source software to be compiled and run, with some emulation of Linux, on top of the Microsoft Windows user API.
  • Interix provides Unix-like functionality as a Windows NT subsystem.



Published - November 2011

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